A technique (for example, see Patent Document 1) has been known that prohibits supply of an aqueous urea solution when an accumulation amount of intermediate products that are produced in a reaction from urea to ammonia in an exhaust passage reaches an upper limit amount in an exhaust purification device that supplies urea to a selective reduction type NOx catalyst (also simply referred to as “NOx catalyst” hereinafter). According to the technique, a reducing agent can be supplied to the NOx catalyst until the accumulation amount of the intermediate products in the exhaust passage reaches the upper limit amount.
Incidentally, a filter for collecting particulate matters (also simply referred to “PM” hereinafter) may be installed in the exhaust passage. In addition, a PM sensor that detects a PM amount in the exhaust is installed for determination on trouble of the filter. It may become difficult to accurately detect the PM amount when the intermediate products attach to an electrode or a cover of the PM sensor. This may decrease accuracy of a determination on filter trouble.